A standard self-tightening drill chuck normally has a chuck body adapted to be mounted on a drill spindle for rotation about an axis and formed with substantially cylindrical front and a rear outer surfaces both centered on the axis and axially offset from each other and with an outwardly open groove between the front and rear surfaces. A tightening sleeve surrounding the body is formed with substantially cylindrical front and rear inner surfaces both centered on the axis and riding on the front and rear surfaces of the body and with an inwardly open groove between the front and rear inner surfaces and forming with the groove of the body an annular generally circular-section passage. The cylindrical surfaces keep the sleeve and chuck body centered on each other. The front part of this sleeve, which is typically made of several pieces, is formed with a rearwardly flared substantially frustoconical seat also centered on the axis. Rollers riding in the grooves support the sleeve on the body so as to permit relative rotation of the body and sleeve about the axis while preventing relative axial displacement. A jaw guide in the sleeve is formed with a plurality of axially forwardly open and angularly spaced slots in which respective jaws that also ride on the seat are axially and radially displaceable.
As described in German patent application P 5134 the seat is formed by a separate sleeve that is threaded onto the front end of the tightening sleeve. Thus the accuracy of the centering of the jaws, which is largely determined by the location of the center of rotation of the frustoconical jaw seat, is in large part determined by the tolerances of the screwthread connection between the jaw-seat sleeve and the tightening sleeve. In order to simplify this construction it has been suggested to form the jaw guide unitarily with the tightening sleeve and to form on the outside of the jaw guide a forwardly tapered frustoconical surface that fits complementary in the jaw seat and, therefore, centers the jaw-seat sleeve. Such a system does not ensure perfectly accurate centering of the jaws, however, so that the bit held thereby rotates offcenter and bores inaccurately.
In another suggested solution the tightening sleeve engages around the roller bearing with a radially inwardly projecting flange on the chuck body. The jawseat sleeve is also threaded with the tensioning sleeve and the jaw guide is separate from the tensioning sleeve and locked axially between the flange of the chuck body and the inner seat surface of the jaw-seat sleeve. This system eliminates the front guide surface for the tightening sleeve so that it also does not accurately center the jaws, and the threads once again also weaken the connection between the jaw-seat and tightening sleeves.